You killed 4 enemies in less than 1 second using the crossbow
For this you will need either Bend Time or Bend Time 2, and Crossbow Reload Speed so your Crossbow automatically reloads after each shot. Once you have that, head to the Lady Boyle's Last Party mission and head across the river via the bridge, and into the building that is next to where your boat landed. On the bottom floor of this building, under the staircase, you will find a group of 6 Weepers just hanging out and waiting to be killed by you. Once you find them, use Bend Time and aim for the head of at least 4 of them.
@2 you are right thats how i got mine purchase stop time find 4 enemies and shoot arrows at the 4 enemies to kill. but be careful not to touch the arrows you shot or the will go flying into enemies and may cause you to not get the cheeve
do it at the Boyle mansion party, should be able to do easy as the guest die easily. and if you are patient, you might get 6 of them close enough and do the 6 kills in 1 sec there too.
also in mission 7 towards the end you have to break some plank to get through then run in near your escape hatch where you have to hold X for it to open and 6 weepers will run in. stop time and shoot :)
I got the 6 in 1 sec with crossbow at boyle mansion during the party but it did not unlock the 4 in 1 sec so i did it where you pick up your gear after you are betrayed. next to your stuff, 4 or 5 weepers will just be standing there, blink down there from the chain, grab your stuff, stop time and shoot them. There i got the 4 in 1 sec.
I found the building on the right side of the boyle party before u enter the mansion was the easiest for this one & 6 kills in 1 sec - I only had slow down time shot 4 arrows & killed them - then reloaded slowed down time & cooked a granade then threw it perfect 6 weepers waiting there to die lol